Michael Fighting
Saint Thomas on Usury
This is an excert only. The whole article can be found at this site.
Summa Theologica: II-II Q78 A1: On the contrary, It is written (Ex. 22:25): "If thou lend money to any of thy people that is poor, that dwelleth with thee, thou shalt not be hard upon them as an extortioner, nor oppress them with usuries."
I answer that, To take usury for money lent is unjust in itself, because this is to sell what does not exist, and this evidently leads to inequality which is contrary to justice. In order to make this evident, we must observe that there are certain things the use of which consists in their consumption: thus we consume wine when we use it for drink and we consume wheat when we use it for food. Wherefore in such like things the use of the thing must not be reckoned apart from the thing itself, and whoever is granted the use of the thing, is granted the thing itself and for this reason, to lend things of this kin is to transfer the ownership. Accordingly if a man wanted to sell wine separately from the use of the wine, he would be selling the same thing twice, or he would be selling what does not exist, wherefore he would evidently commit a sin of injustice. In like manner he commits an injustice who lends wine or wheat, and asks for double payment, viz. one, the return of the thing in equal measure, the other, the price of the use, which is called usury.
